During the past years, the interest in using mobile and landline/wireline computing devices in day-to-day communications has increased. Desktop computers, workstations, and other wireline computers currently allow users to communicate, for example, via e-mail, video conferencing, and instant messaging (IM). Mobile devices, for example, mobile telephones, handheld computers, personal digital assistants (PDAs), etc., also allow the users to communicate via e-mail, video conferencing, IM, and the like. Mobile telephones have conventionally served as voice communication devices, but through technological advancements they have recently proved to be effective devices for communicating data, graphics, etc. Wireless and landline technologies continue to merge into a more unified communication system, as user demand for seamless communications across different platforms increases.
Considering wireless devices, one feature that has been introduced over the years is positioning which enables the location of an end user device to be determined by the network. For example, third generation cellular systems may be equipped with one or more of a number of different positioning methods which enable location services to the cellular subscribers. Some positioning methods currently in use include: Cell identity (Cell-ID) positioning; Enhanced cell identity (Ecell-ID) positioning; Assisted Global Positioning System (A-GPS) positioning; Observed downlink time difference of arrival—with idle periods in the downlink (OTDOA-IPDL) positioning; and Uplink time difference of arrival (UTDOA) positioning. Each of these positioning methods is briefly described below.
Cell-ID positioning determines the cell to which a user equipment (UE) is currently connected and uses that cell location as a rough approximation of the UE's current position. The position of the user is then determined with some amount of cell granularity. Although somewhat imprecise, the Cell-ID positioning method is considered to be the backbone of cellular positioning systems since it is always available for a UE that is connected to the RAN and is technically simple to implement.
Ecell-ID positioning augments the Cell-ID positioning technique with auxiliary information that narrows down the within the cell where the UE is located. Auxiliary information that can be used, for example, from a wideband code division multiple access (WCDMA) system, is the round trip time (RTT) measurement. The RTT measurement determines the travel time from the radio base station (RBS) to the UE and back. Using the speed of light, the distance from the known position of the RBS to the UE can be calculated, which results in a ring around the RBS within which the UE is located. The thickness of the ring is determined by the measurement uncertainty. The Ecell-ID method recognizes that the UE is located both in the cell and, more specifically, in the ring around the RBS and determines that the UE is located within a zone described by the intersecting areas of these two geographical regions.
A-GPS positioning is an enhancement of the United States' military global positioning system (GPS). GPS reference receivers attached to, e.g., a cellular communication system, collect assistance data, that, when transmitted to GPS receivers in terminals connected to the cellular communication system, enhances the performance of the GPS terminal receivers. Typically, A-GPS accuracy can become as accurate as within 10 meters without differential operation. The accuracy decreases in dense urban areas and indoors, where the sensitivity of the GPS receivers is not high enough for detection of the relatively weak signals from the GPS satellites. The method is relatively accurate and meets the North American emergency positioning E-911 requirements of 50 meters for 67% of all positionings and 150 meters for 95% of all positionings A concern with the A-GPS positioning technique is the limited indoor coverage due to the low ranging signal strengths that are typically obtained at ground level.
OTDOA-IPDL positioning is similar to A-GPS in that it relies on time difference of arrival measurements. However, the OTDOA-IPDL method uses UE measurements of pilot radio, or Common Pilot Channel (CPICH) in WCDMA, signals transmitted from several RBSs. The measurement results are signaled to the RNC, where a hyperbolic trilateration method is used for calculation of the position of the UE. In order to enhance the hearability of the RBSs' signals in the UE, it is also possible to use IPDL to attenuate the transmissions from the RBS to which the UE is connected. This reduces the interference and hence enhances the hearability of other RBSs. Additionally, OTDOA-IPDL theoretically provides better indoor coverage than A-GPS.
Another positioning method is UTDOA positioning. UTDOA, like OTDOA-IPDL, is similar to A-GPS in that UTDOA also relies upon time difference of arrival measurements. However, the UTDOA method also uses RBS or separate location measurement unit (LMU) measurements of signals transmitted from the positioned UE. The transmitted signal is detected in a number of RBSs or LMUs, after which the measured results are signaled to a positioning node where the position of the UE is determined by a trilateration method. In order to detect the time of arrival from measurements of opportunity from the UE, a reference signal first needs to be created in a master LMU or master RBS. This can be done by decoding the signal followed by reconstruction of the chip stream that then forms the reference signal. When compared to A-GPS, UTDOA positioning typically provides better indoor coverage but typically provides inferior outdoor accuracy.
One issue with terrestrial time difference of arrival methods, e.g., OTDOA-IPDL and UTDOA, involves the receiver sensitivity when positioning is considered. Theoretically, these methods can provide a 3D position from four time of arrival measurements, i.e., the equivalent of three time difference of arrive measurements or pseudo-measurements. However, radio propagation conditions are less beneficial for these terrestrial time difference of arrival methods when compared with A-GPS since their ranging signals propagate along the surface of the Earth, whereas A-GPS signals propagate from above. These terrestrial positioning methods therefore suffer more from non-line-of-sight (LOS) propagation and multipath propagation. This results in outlier measurements, whose suppression requires the availability of excess detections, i.e., detections from significantly more than the minimum number of RBSs. In practice, to achieve a useful positioning accuracy, at least 6-8 RBSs need to be detected in the UE when OTDA-IPDL positioning is used. For UTDOA positioning at least 6-8 RBSs need to detect the UE transmissions in order to obtain useful position estimates in practical environments.
The consequence of the above, i.e., sensitivity and propagation path issues, is that more remote RBSs need to be detected for OTDOA-IPDL or to do the detecting for UTDOA. This means that lower signal strengths need to be detected with high probability. Calculations typically show that signals need to be detected down to about −40 dB C/I. Further, the pre-detection step needs to enhance the signal to about 11-13 dB C/I in order to achieve a sufficiently low false rate. In essence, the processing gain for positioning purposes in any CDMA system needs to be substantially 50-55 dB for terrestrial positioning to be useful. This processing gain is significantly more than what is needed for other services, which may lead one to consider that positioning sensitivity requirements need to be assessed at the definition phase of the air-interface.
Accordingly, for new generations of mobile communication systems, as well as to improve currently used mobile communication systems, methods and systems to improve the hearability of signals used for positioning purposes are desirable.